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MFLAG
Pin
+100
(AINP
AINN)
-
-
100
0
Time
V
(%ofF
ull-Scale)
IN
f
MOD
/2
MFLAG
Pin
100% FS
AINN
AINP
P
Q
IABSI
å
SBAS418I – SEPTEMBER 2007 – REVISED MARCH 2015
Modulator Over-Range (continued)
9.11 Modulator Over-Range Detection
(MFLAG)
the normal range, the modulator requires up to 12
The ADS1282 has a fast-responding over-range
modulator clock cycles (f
MOD
) to exit saturation and
detection that indicates when the differential input
return to the linear region. The digital filter requires an
exceeds 100% or –100% full-scale. The threshold
additional 62 conversions for fully settled data (linear
tolerance is ±2.5%.The MFLAG output asserts high
phase FIR).
when in an over-range condition. As
and
In the extreme case of over-range, either input is
illustrate, the absolute differential input is
overdriven, exceeding the voltage of either analog
compared to 100% of range. The output of the
supply voltage plus an internal ESD diode drop. The
comparator is sampled at the rate of f
MOD
/2, yielding
internal diodes begin to conduct and the signal on the
the MFLAG output. The minimum MFLAG pulse width
input is clipped. When the input overdrive is removed,
is f
MOD
/2.
the diodes recover quickly. Keep in mind that the
input current must be limited to 100mA peak or 10mA
continuous if an overvoltage condition is possible.
9.10 Modulator Input Impedance
The modulator samples the buffered input voltage
with an internal capacitor to perform conversions. The
charging of the input sampling capacitor draws a
transient current from the PGA output. The average
value of the current can be used to calculate an
Figure 33. Modulator Over-Range Block Diagram
effective input impedance of R
EFF
= 1/(f
MOD
× C
S
).
Where:
f
MOD
= Modulator sample frequency (CLK / 4)
C
S
= Input sampling capacitor (17pF, typ)
The resulting modulator input impedance for CLK =
4.096MHz is 55k
Ω
. The modulator input impedance
and the PGA output resistors result in a systematic
gain error of –1%. C
S
can vary ±20% over production
lots, affecting the gain error.
Figure 34. Modulator Over-Range Flag Operation
16
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